The development of HIV vaccines and other prevention strategies relies on the use of nonhuman primates in preclinical studies to advance the development of effective AIDS vaccine candidates and to advance development of effective topical microbicides and other prevention modalities or immune-based therapies. The NIAID Simian Vaccine Evaluation Unit (SVEU) contracts shall provide nonhuman primate resources that primarily support preclinical evaluation of AIDS vaccines. The SVEUs conduct studies in support of vaccines being developed by a wide range of investigators. These studies complement NIAID-supported basic vaccine research and vaccine evaluation studies funded through R01 (investigator-initiated research) grants, HIVRAD (HIV Vaccine Research and Design Program) grants, IPCAVD (Integrated Preclinical-Clinical AIDS Vaccine Development) grants, and NHP Consortium awards. The SVEUs also perform studies of candidate vaccines offered by companies or other researchers. Exposure to the human Immunodeficiency virus -1 (HIV-1) does not always result in infection. Indeed there are reports of multiply, HIV-exposed seronegative (HESN) persons who remain uninfected. Thus far, only one determinant has been found to be associated with protection, the presence of the CCR5?32 gene in some individuals. Other factors to explain the lack of infection have not been satisfactorily demonstrated. Consistent with the finding of HESNs, nonhuman primates challenged multiple times with simian immunodeficiency virus (SIV), but remaining uninfected, are commonly observed. Therefore, the nonhuman primate continues to be the best model in which to examine potential genetic and immunologic factors that contribute to resistance of infection. Understanding the mechanism(s) and defining the correlate(s) of protection would have a major impact on HIV transmission.